The invention relates generally to the field of electrophysiological analysis of the human body, with a view for example to detecting medical conditions. The invention is particularly applicable to evaluating sudomotor function in the human body.
The applicant previously proposed in the patent FR 2 912 893 an electrophysiological analysis system comprising a series of electrodes intended to be placed in different regions of a subject's body, a direct current voltage source, suitable for generating adjustable direct current pulses, and a switching circuit, arranged to selectively connect a pair of so-called active electrodes to the voltage source, said active electrodes forming an anode and a cathode, and to connect at least one other electrode with high impedance. The voltage applied by the voltage source to the electrodes is suitable for generating in the outer layer of the skin an electrophysiological current wherein the study of certain characteristics may indicate the existence of medical conditions or predisposition to medical conditions.
In particular, and with reference to FIG. 1, the current generated in the skin by applying a voltage is suitable for studying the electrical behaviour of the subject's sweat glands, by measuring the conductance of these glands. This conductance, corresponding to the slope of the voltage-current curve, for the lowest voltages (see patent application FR1160601), varies according to the subject's state of health. In this way, for example, a low voltage-current slope may be an indication, in a diabetic subject, of diabetic neuropathy, as described in the document by Gin H, et al. “Non-invasive and quantitative assessment of sudomotor function for peripheral diabetic neuropathy evaluation. Diabetes Metab (2011), doi:10.1016/j.diabet.2011.05.003”.
It has also been observed that the voltage-current curve has, for the lowest voltage levels, a linear portion, followed by a non-linear detachment at higher levels, see FIG. 1. A corresponding detachment takes place for the skin conductance according to the voltage level applied to the body. This detachment does not occur in the same way for different types of subjects, according to any disease from which they suffer, such that the study of this detachment in a subject could be suitable for inferring information on the subject's state of health. However, this detachment does not occur, from one subject to another, on applying the same voltage or for the same current; on the contrary, the inventors observed that the occurrence thereof was dependent on the potential reached by the body and more specifically the difference in potential on either side of the gland, i.e. between the electrode and the body. It is thus suitable to determine this detachment, for all subjects, at the same difference in potential between the electrode and the body.
However, current systems are merely suitable for having access to the difference in potential between the electrode and the potential reached by the body and may not in principle apply this difference in potential, such that the measurements are not made for a constant difference in potential. These systems are thus not suitable for making full use of the voltage-current curve in terms of the detachment thereof to obtain information on the subject's state of health.